Thermostructural composites are known for their good mechanical properties and their ability to maintain these properties at high temperature. They comprise carbon/carbon (C/C) composites formed from a carbon-fiber reinforcement densified by a carbon matrix and ceramic matrix composites (CMC) formed from a reinforcement made of refractory (carbon or ceramic) fibers densified by an at least partly ceramic matrix. Examples of CMCs are C/SiC composites (carbon fiber reinforcement and silicon carbide matrix), C/C—SiC composites (carbon fiber reinforcement and matrix comprising a carbon phase, generally as close as possible to the fibers, and a silicon carbide phase) and SiC/SiC composites (silicon carbide reinforcing fibers and silicon carbide matrix). An interphase layer may be interposed between reinforcing fibers and matrix in order to improve the mechanical integrity of the material.
Because of their properties, thermostructural composites find applications in various fields, to produce components that have to be subjected to high thermomechanical stresses, for example in the aeronautical and space fields. However, in such fields, although the composites have already allowed components to be produced with good mechanical strength with a lower weight than metals for example, the mass of these components is not insignificant and has an influence on the performance of the systems. Consequently, there is still a demand for lighter thermostructural components or assemblies, but without the mechanical properties suffering.
One solution consists in forming lightened thermo-structural structures, for example of the cellular type, such as honeycomb structures, rather than solid structures, so as to retain good mechanical strength but with less material. This implies being able to manufacture thin-walled thermostructural composite structures of complex shape.
When it is desired to obtain a component of a particular shape from a thermostructural composite, the procedure is generally as follows. A preform is produced by forming the fibrous reinforcement. The preform, optionally kept in shape by means of a tool, is then densified. A component made of a rigid composite with a defined shape is thus obtained.
The usual densification processes are the liquid process and the gas process.
The liquid process consists in pre-impregnating the preform with a liquid composition containing a precursor of the material of the matrix. The precursor is usually in the form of a polymer, such as a resin, optionally diluted in a solvent. The conversion of the precursor into carbon or ceramic is carried out by heat treatment, after removal of the optional solvent and crosslinking of the polymer. Several successive impregnation cycles may be carried out in order to achieve the desired degree of densification. To give an example, liquid carbon precursors may be resins with a relatively high coke content, such as phenolic resins, whereas liquid ceramic precursors, especially those based on SiC or Si3N4, may be resins of the polycarbosilane (PCS) or polytitanocarbosilane (PTCS) type.
The gas process consists of chemical vapor infiltration. The fibrous preform corresponding to a component to be produced is placed in a furnace into which a reactive gas phase is introduced. The pressure and the temperature in the furnace and the composition of the gas phase are chosen so as to allow the gas phase to diffuse into the pores of the preform in order to form therein the matrix by deposition, in contact with the fibers, of a solid material resulting from the decomposition of a constituent of the gas phase or from a reaction between several constituents. To give an example, gaseous carbon precursors may be hydrocarbons, that give carbon by cracking, and a gaseous ceramic precursor, especially SiC precursor, may be methyltrichlorosilane (MTS), that gives SiC by decomposition of the MTS. In this case, the optional tools used to keep the preform in a defined configuration must be suitable for allowing the gases to pass into the preform to be treated.
However, these manufacturing methods do not allow complex components having very thin parts of precise shape and dimensions to be obtained directly.
Detailed parts may be machined in the component after densification. In this case, to maintain sufficient stiffness in the thin parts, the composite used must be very strong, that is to say have a high density. For this purpose, it is possible to use composites with a ceramic matrix, such as silicon carbide, which gives the composite a high stiffness even when it has a small thickness. However, the weight saving obtained by removing material is limited by the required density of the composite used.
Moreover, the use of very hard materials such as silicon carbide requires the use of specific tools that allow precision machining of such materials, and this increases the difficulties and the costs of manufacture.